In a large number of industrial applications, pressure measuring transducers are exposed to drastic, sometimes very abrupt, fluctuations in temperature. Additionally, in certain industries, e.g. in the pharmaceutical industry, there are very high hygienic requirements. Normally in these industries, cleaning processes are used, in which the pressure sensors can be exposed to severe fluctuations in temperature. Examples of such cleaning processes are the so-called Cleaning in Place (CIP) or Sterilization in Place (SIP) processes, in which the containers are cleaned or sterilized without first removing the measuring devices or transducers. In such cases, for example, a spray head is arranged in the container for delivering cleaning chemicals and water or steam, and for rinsing, washing, or boiling the container as needed. Depending on the application, temperatures can range from e.g. −20° C. up to 200° C., for example.
Such a broad temperature range creates problems with respect to accuracy of the pressure measuring transducers. Pressure measuring transducers are usually assembled from various components of different materials, whose different thermal expansion as a function of temperature can lead to stressing, warping, and in the worst case, even deformation of the components.
Today, a number of pressure measuring transducers have diaphragm seals, which transfer a pressure to be measured, acting on a separating diaphragm, via a pressure-transferring liquid, to a pressure-sensitive, measuring element. Pressure-transferring liquids have coefficients of thermal expansion which work in such a way that the volume of the liquid contained in the pressure measuring transducer changes with the temperature. This leads to measurement errors. Furthermore, pressure-transferring liquids are never, or only reluctantly, used in certain applications in which there are high safety and/or hygiene requirements, because the danger exists that the liquid can leak out, if the pressure measuring transducer is damaged. In these applications, so-called “dry pressure transducers,” i.e. such that do without a pressure-transferring liquid, are preferably used.
Today, semiconductor pressure transducers are preferably used. An especially preferred example is represented by sapphire carriers having integrated silicon sensors, e.g. silicon strain resistors or resistor bridge circuits. Such sensors are known from “Silicon on Sapphire” technology (SOS technology). They offer the advantage that they can be used at very low and also at very high temperatures.
In traditional silicon technology, the silicon sensors are placed on silicon carriers, and e.g. isolated by PN-junctions. However, this isolation is only effective at temperatures below approximately 150° C. In contrast, pressure sensors assembled with SOS technology offer the advantage that sapphire is a dielectric, which guarantees a good isolation of the integrated sensors at temperatures of up to 350° C.
Sapphire is mechanically highly stable, and has a crystal structure, which is compatible with that of silicon.
Sapphire carriers with integrated silicon sensors can be used in a very broad temperature range, and can also withstand sudden, drastic fluctuations in temperature. However, problems occur when, e.g., these pressure sensors are used in a measuring transducer made of stainless steel. Sapphire has a coefficient of thermal expansion of 8×10−6/K, as contrasted with that of stainless steel, which is 16×10−6/K.
Today, there are pressure measuring transducers, in which silicon sensors placed on sapphire are mounted on a separating diaphragm made of titanium. Titanium has a coefficient of thermal expansion which corresponds to that of sapphire. Titanium provides very high-value, but is also a very expensive material.
Another requirement of pressure measuring transducers, especially those in applications with high hygienic demands, is that of front-flushness. This means that the pressure measuring transducer must terminate in a plane facing the process, and especially may have no gaps, cavities and/or undercuts, into which the medium contained at the measuring site, and whose pressure is to be measured, can penetrate.